1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer, etc. employing an intermediate transfer body.
2. Related Background Art
An image forming apparatus of full color including four colors shown in FIG. 5, such as a copying machine, a laser beam printer, etc. employs an intermediate transfer body 5 as a second image bearing body outside a photosensitive drum 1 serving as a first image bearing body. It has been known that the image forming apparatus using the intermediate transfer body 5 has very little color misregistration of a color image formed by superposing toner images having a plurality of colors.
In the image forming apparatus illustrated in FIG. 5, the surface of the photosensitive drum 1 is a first image bearing body which rotates in the direction shown by an arrow mark R1 and is uniformly charged by a charging roller 2 so as to have a prescribed polarity and a prescribed potential. The surface of the charged photosensitive drum 1 is scanned and exposed with a laser beam L corresponding to yellow by an exposure device 3 to form an electrostatic latent image thereon. This electrostatic latent image has yellow toner adhered thereto by the yellow developer 4a of a developing device 4 and is developed as a toner image. The yellow toner image is primarily transferred to the surface of the intermediate transfer body 5 rotating in the direction of an arrow mark R5 through a primary transfer nip T1. The toner which is not transferred to the intermediate transfer body 5 but remains on the surface of photosensitive drum 1 after the primary transfer is removed by a cleaning device 7a. 
The same image forming processes as those mentioned above are also repeated for three colors other than yellow, that is to say, magenta, cyan and black, so that the toner images of four colors are superposed on the intermediate transfer body 5 to form color images.
The color images are simultaneously secondarily transferred to a transfer material P through a secondary transfer nip T2 by a transfer roller 8 to which secondary transfer bias applied power 10b is applied. The transfer material P to which the color images are secondarily transferred is conveyed to a fixing device 9. In the fixing device 9, the transfer material P is heated and pressed so that the color images are fixed to the surface thereof. On the other hand, after the transfer of the toner image, toner remaining after the secondary transfer which is not transferred to the transfer material P but remains on the surface of the intermediate transfer body 5 is removed by a cleaning device 7b. 
Referring to FIG. 5, reference characters 4b, 4c and 4d respectively designate the developers of magenta, cyan and black in an order described above. Further, 10a is a controller for controlling the secondary transfer bias applied power 10b. 
In the above described image forming apparatus, the primary transfer of the toner image to the intermediate transfer body 5 from the photosensitive drum 1 is repeated four times, then, the toner images of four colors are superposed on the intermediate transfer body 5 and these toner images (color images) are simultaneously secondarily transferred onto the transfer material P. Therefore, this image forming apparatus is referred to as a multiple transfer type image forming apparatus.
In the multiple transfer type image forming apparatus mentioned above, however, the toner images have been superposed on the intermediate transfer body 5 and repeatedly transferred thereto, hence there has sometimes appeared a phenomenon called a retransfer that the toner image which has been already transferred to the intermediate transfer body 5 has been retransferred to the photosensitive drum 1 during the transfer process of a next color. If this retransfer phenomenon is generated, the density of an image will be lowered and a defective image will be undesirably formed.
The above retransfer phenomenon is typically generated under conditions mentioned below. As the number of times of primary transfer is increased after the toner image is transferred to the intermediate transfer body 5 from the photosensitive drum 1, this retransfer phenomenon is more liable to be generated, because the charged polarity of toner is apt to be reversed due to a plurality of times of transfers. For example, in the above conventional case, since the yellow toner image corresponds to a first color, the yellow toner image is retransferred to the photosensitive drum during the transfer of second to fourth colors.
Further, the retransfer phenomenon is apt to be generated when a reversal developing method is employed and there is no toner on the photosensitive drum 1 upon generation of a retransfer phenomenon: that is to say, this phenomenon tends to be generated on the white background part or the shadow portion of the photosensitive drum. Specifically, the potential difference of the shadow portion of the photosensitive drum 1 is large relative to transfer voltage applied to the intermediate transfer body and large transfer current is supplied thereto, so that the charged polarity of toner is readily reversed.
Further, the retransfer phenomenon is liable to be generated in the environment of high humidity in which large transfer current is apt to be supplied.
Furthermore, the lower the resistance value of the intermediate transfer body 5, the more easily the retransfer phenomenon is generated.
To summarize the above description, the retransfer phenomenon indicates that the transfer current is excessively supplied so that the charged polarity of toner is reversed to an ordinary charged polarity, and therefore, the toner returns to the photosensitive drum 1.
Accordingly, it is an object of the present invention to provide an image forming apparatus capable of preventing a retransfer phenomenon in which a toner image temporarily transferred to an intermediate transfer body returns to an image bearing body.
It is another object of the present invention to provide an image forming apparatus capable of preventing a toner image of a certain color transferred to an intermediate transfer body from returning to an image bearing body upon transfer of toner images of other colors after the first color to the intermediate transfer body.
It is still another object of the present invention to provide an image forming apparatus capable of decreasing the potential difference between the potential of a shadow portion and transfer voltage by decreasing the thickness of the layer of an image bearing body and reducing the potential of the shadow portion of the image bearing body under the environment of high humidity.